Single chain atibody consists of peptides, genetically analyzed from monoclonal antibody, and is reactive with the corresponding antigens. The advantages of single chain antibodies are (1) no immunogenecity to humans, (2) chemically produced, and (3) responding to the same antigen with the original monoclonal antibody.Single chain antibody was obatained from the murine antibody reactive with ovarian cancer antigen CA125, and iodinated with radioiodine-125. However I-125 labeled antibody was not stable and its binding to CA125 antigen was not confirmed. Binding of heavy chain and light chain remains to be studied.In contrast, chimeric antibody which reserves the antigen-binding sites of murine antibody was successfully labeled with technetium-99m and I-125.Chimeric antibody reactive with NCA-95 bindis to human leukocytes and colorectal cancer cells expressing carcinoembryonic antigen (CEA). After getting the permission of clinical use from the ethical committee, Tc-99m labeled was safely administered and was very useful in patients with a plastic anemia, myelofibrosis and leukemia. Tc-99m labeled chimeric antibody to NCA-95 would be also clinically useful for the localization of infectious sites and abscess.